Proteins shape up in the ribosome

Proteins consist of linear chains of amino acids. These chains must fold into complex three-dimensional shapes to become functional. Holtkamp et al. “watched” how a small helical protein folds as it is being synthesized by the ribosome. The lengthening polypeptide passes out through the ribosome exit tunnel where folding starts. The initially compact structure quickly rearranges into a native three-dimensional structure as the polypeptide emerges from the tunnel.

Abstract

Protein domains can fold into stable tertiary structures while they are synthesized on the ribosome. We used a high-performance, reconstituted in vitro translation system to investigate the folding of a small five-helix protein domain—the N-terminal domain of Escherichia coli N5-glutamine methyltransferase HemK—in real time. Our observations show that cotranslational folding of the protein, which folds autonomously and rapidly in solution, proceeds through a compact, non-native conformation that forms within the peptide tunnel of the ribosome. The compact state rearranges into a native-like structure immediately after the full domain sequence has emerged from the ribosome. Both folding transitions are rate-limited by translation, allowing for quasi-equilibrium sampling of the conformational space restricted by the ribosome. Cotranslational folding may be typical of small, intrinsically rapidly folding protein domains.